CHAPTER 2
2
Overview
PDF is a file format for representing documents in a manner independent of the
application software, hardware, and operating system used to create them and of
the output device on which they are to be displayed or printed. A
PDF document
consists of a collection of
objects
that together describe the appearance of one or
more
pages,
possibly accompanied by additional interactive elements and higher-
level application data. A
PDF file
contains the objects making up a PDF docu-
ment along with associated structural information, all represented as a single self-
contained sequence of bytes.
A document’s pages (and other visual elements) can contain any combination of
text, graphics, and images. A page’s appearance is described by a PDF
content
stream,
which contains a sequence of
graphics objects
to be painted on the page.
This appearance is fully specified; all layout and formatting decisions have al-
ready been made by the application generating the content stream.
In addition to describing the static appearance of pages, a PDF document can
contain interactive elements that are possible only in an electronic representation.
PDF supports
annotations
of many kinds for such things as text notes, hypertext
links, markup, file attachments, sounds, and movies. A document can define its
own user interface; keyboard and mouse input can trigger
actions
that are speci-
fied by PDF objects. The document can contain
interactive form
fields to be filled
in by the user, and can export the values of these fields to or import them from
other applications.
Finally, a PDF document can contain higher-level information that is useful for
interchange of content among applications. In addition to specifying appearance,
a document’s content can include identification and logical structure information
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